"The cause of the flood was a series of cloud-bursts resulting from heavy rain-bearing clouds striking against the mountains and being deflected upward." 

-1922 USGS Flood Report

"Before the night is over, that cloud will cause great damage."

-R.K. Potter of Pueblo, June 2, 1921

This cloudburst image may reflect what some Puebloans witnessed on the plains west of town the night before the flood: a massive cloud that unleashed waves of water into the Arkansas drainage. Again, from the USGS report: 

Between June 2 and June 5, 1921, heavy rains of an intensity to justify the term "cloud-bursts" in the foothill regions of the Arkansas River Valley in Colorado caused the severest flood in the valley since its settlement. . . . The flood was remarkable for the very small area covered by the rainfall that was its chief cause, and for the swift rise of the river to an unprecedented stage and its almost equally rapid fall. The swift rise and fall indicated very great flood discharges of the tributary streams, which drain a mountainous country of steep slopes. The total discharge of the main flood was less than 90,000 acre-feet.

*note:  A cloudburst is a sudden severe rainstorm that falls for a short period of time that is limited to a small geographical area. Cloudbursts are generally associated with thunderstorms, the air currents that rush upwards in a rainstorm hold a large amount of water. If the currents suddenly stop, the entire amount of rain falls onto a small area with huge force. This is due to a rapid condensation of the clouds. Cloudbursts occur most often in desert and mountain areas, and in interior regions of continental landmasses. If a cloudburst happens the results are usually catastrophic.

The USGS produced the above map in their 1922 assessment of the flood. Blue shaded areas represent areas of heavy rainfall that caused the Arkansas to overflow its banks in early June of 1921.